A workflow is the process you use to prioritize, plan and work off your to-dos. 

There are many ways you can go about tackling your work and having a process (a repeatable set of steps) makes this very efficient.

One category of workflows is the day planning workflows. They involve making some kind of plan for your day either the night before or first thing in the morning.

There are many different variations of a day planning workflow. 

The simplest day planning workflow is just making a list of the things you want to do today and then working off that list.
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Another example of a variation on this basic theme  is the 1-2-3 workflow where each day you plan to do one big task, 2 medium sized tasks and 3 small ones.

Instead of making a simple list you could also schedule tasks into a calendar like timeline specifying how long each task will take. This day planning type workflow is less flexible than the others, but the reduction in flexibility can be beneficial for some. 

A very popular workflow is GTD  (Getting Things Done by David Allen). Instead of making a day plan you always have a list of next actions you can do. The next action list can be filtered by context and based on what you currently want to work on.

Batching is another methodology that can be used in workflows. You group tasks with a similar activity (e.g. making phone calls, writing emails) and then tackle them together.

There are a ton of workflows that just deal with prioritization. 

The Eisenhower matrix is famous for categorizing your to-dos based on importance and urgency for example. 

Ultimately a workflow is the combination of all the methods you use in getting your work done. How you prioritize your list, how you review your workload, the plans you make and how you work off those plans. 

The key here is to realize that you have to have some kind of system behind managing your tasks and time. Just writing things down is not a system. It's just a good start. :) 

What works for you is dependent on your brain and the type of work you do. Finding a good workflow takes time. But there is a lot of value in the experimentation itself. Experimenting with workflows raises self awareness and increases productivity. 

Marvin is designed to make workflow experimentation simple and flexible.